1. Membrane Technology
Common membrane separation techniques include microfiltration, nanofiltration, ultrafiltration, and reverse osmosis. Since membrane technology does not introduce additional impurities during the treatment process, it can effectively separate macromolecular and small-molecule substances, making it widely used for the recovery of various macromolecular raw materials.
For example, ultrafiltration technology is used to recover polyvinyl alcohol slurry from dyeing and printing wastewater. The primary challenges in the engineering application and promotion of membrane technology include high membrane costs, short membrane lifespan, susceptibility to contamination, and scaling and blockage. With the advancement of membrane production technology, membrane technology will find increasing application in wastewater treatment.
2. Iron-carbon microelectrolysis treatment technology
Iron-carbon microelectrolysis is an effective process for treating wastewater based on the Fe/C galvanic cell reaction principle, also known as internal electrolysis or iron filings filtration. The iron-carbon microelectrolysis method is a comprehensive effect of electrochemical oxidation-reduction, electrochemical pair-mediated flocculation enrichment, and the coagulation of electrochemical reaction products, adsorption of newly formed flocs, and bed filtration. Among these, the primary mechanisms are oxidation-reduction, electro-adsorption, and coagulation.
When iron filings are immersed in wastewater containing a large amount of electrolytes, numerous micro-galvanic cells are formed. When charcoal is added to the iron filings, larger galvanic cells are formed at the interface between the iron filings and charcoal particles. This results in the iron filings being corroded not only by the micro-galvanic cells but also by the larger galvanic cells, thereby accelerating the electrochemical reaction.
This method has numerous advantages, including a wide range of applications, excellent treatment efficiency, long service life, low cost, and convenient operation and maintenance. It uses waste iron filings as raw material and does not consume electrical resources, embodying the principle of “treating waste with waste.” Currently, iron-carbon microelectrolysis technology has been widely applied in the treatment of wastewater from industries such as textile dyeing, pesticides/pharmaceuticals, heavy metals, petrochemicals, and oil separation, as well as in the treatment of leachate from landfills, achieving good results.
3. Fenton and Fenton-like oxidation methods
Typical Fenton reagents are Fe²⁺-catalyzed decomposition of H₂O₂ to produce ·OH, thereby initiating the oxidation and degradation of organic matter. However, the Fenton method requires a long treatment time, uses a large amount of reagents, and excess Fe²⁺ can increase the COD in the treated wastewater and cause secondary pollution.
In recent years, researchers have introduced ultraviolet and visible light into the Fenton system and explored the use of other transition metals to replace Fe²⁺. These methods significantly enhance the Fenton reagent's ability to oxidize and degrade organic matter, reduce reagent consumption, and lower treatment costs, collectively referred to as Fenton-like reactions.
The Fenton method has mild reaction conditions, simple equipment, and a wide range of applications; it can be used as a standalone treatment technology or combined with other methods, such as coagulation precipitation, activated carbon, or biological treatment, as a pretreatment or advanced treatment method for difficult-to-degrade organic wastewater.
4. Ozone oxidation
Ozone is a strong oxidizing agent that reacts rapidly with reduced pollutants, is convenient to use, and does not produce secondary pollution. It can be used for wastewater disinfection, decolorization, deodorization, organic matter removal, and COD reduction. However, the ozone oxidation method is costly when used alone, and its oxidation reaction is selective, with poor oxidation effects on certain halogenated hydrocarbons and pesticides.
Therefore, in recent years, combined technologies aimed at improving ozone oxidation efficiency have been developed. Among these, combinations such as UV/O₃, H₂O₂/O₃, and UV/H₂O₂/O₃ not only enhance oxidation rates and efficiency but also oxidize organic compounds that are difficult to degrade when ozone acts alone. Since ozone has low solubility in water and low production efficiency with high energy consumption, increasing ozone solubility in water, improving ozone utilization efficiency, and developing high-efficiency, low-energy ozone generators have become key research directions.
5. Magnetic separation technology
Magnetic separation technology is a novel water treatment technique developed in recent years that utilizes the magnetic properties of impurity particles in wastewater for separation. For non-magnetic or weakly magnetic particles in water, magnetic inoculation technology can be employed to impart magnetic properties to them.
Magnetic separation technology is applied in wastewater treatment through three methods: direct magnetic separation, indirect magnetic separation, and microbial-magnetic separation.
Currently, the magnetic modification technologies being researched primarily include magnetic agglomeration technology, iron salt coprecipitation technology, iron powder method, and ferrite method. Representative magnetic separation equipment includes disc magnetic separators and high-gradient magnetic filters. At present, magnetic separation technology is still in the laboratory research stage and cannot yet be applied to actual engineering practice.
6. Plasma Water Treatment Technology
Low-temperature plasma water treatment technology, including high-voltage pulse discharge plasma water treatment technology and glow discharge plasma water treatment technology, utilizes discharge to directly generate plasma in an aqueous solution or introduces active particles from gas discharge plasma into water, enabling the complete oxidation and decomposition of pollutants in water.
Direct pulse discharge in aqueous solutions can be operated at ambient temperature and pressure. Throughout the discharge process, no catalyst is required to generate in-situ chemically oxidative species that oxidize and degrade organic matter. This technology is economical and effective for treating low-concentration organic matter. Additionally, the reactor configuration for pulse discharge plasma water treatment technology can be flexibly adjusted, the operation process is simple, and maintenance costs are relatively low. Due to the discharge equipment, the energy utilization efficiency of this process for degrading organic matter is relatively low, and the application of plasma technology in water treatment is still in the research and development stage.
7. Electrochemical (Catalytic) Oxidation
Electrochemical (catalytic) oxidation technology directly degrades organic matter through anodic reactions or produces oxidizing agents such as hydroxyl radicals (˙OH) and ozone through anodic reactions to degrade organic matter.
Electrochemical (catalytic) oxidation includes two-dimensional and three-dimensional electrode systems. Due to the micro-electrolytic effect of three-dimensional electrode systems, they are currently highly favored. Three-dimensional electrodes involve filling granular or other particulate working electrode materials between the electrodes of traditional two-dimensional electrolytic cells, charging the surface of the filled materials to form a third electrode, and enabling electrochemical reactions to occur on the surface of the working electrode materials.
Compared to two-dimensional flat electrodes, three-dimensional electrodes have a much larger specific surface area, increasing the surface-to-volume ratio of the electrolytic cell. They can provide a higher current intensity at a lower current density, with smaller particle spacing and higher mass transfer rates, resulting in high spatiotemporal conversion efficiency, thus achieving high current efficiency and excellent treatment performance. Three-dimensional electrodes can be used to treat domestic wastewater, pesticides, dyes, pharmaceuticals, phenol-containing wastewater, and other difficult-to-degrade organic wastewater, metal ions, and landfill leachate.
8. Radiation Technology
Since the 1970s, with the development of large cobalt sources and electron accelerator technology, issues related to radiation sources in radiation technology applications have gradually been improved. Research on using radiation technology to treat pollutants in wastewater has attracted attention and importance from countries worldwide.
Compared to traditional chemical oxidation, the use of radiation technology to treat pollutants does not require the addition of chemical reagents or only requires a small amount, avoiding secondary pollution. It offers advantages such as high degradation efficiency, fast reaction speed, and thorough degradation of pollutants. Additionally, when ionizing radiation is combined with catalytic oxidation methods such as oxygen or ozone, a “synergistic effect” is produced. Therefore, radiation technology for pollutant treatment is a clean and sustainable technology, listed by the International Atomic Energy Agency as one of the primary research directions for the peaceful use of atomic energy in the 21st century.
9. Photochemical Catalytic Oxidation
Photochemical catalytic oxidation technology has evolved from photochemical oxidation and, compared to photochemical methods, possesses stronger oxidative capacity, enabling more thorough degradation of organic pollutants. Photochemical catalytic oxidation involves photochemical degradation under the presence of a catalyst, where oxidizing agents generate highly reactive free radicals under light irradiation.
Catalysts include TiO₂, ZnO, WO₃, CdS, ZnS, SnO₂, and Fe₃O₄, among others. They are classified into homogeneous and heterogeneous types. Homogeneous photocatalytic degradation uses Fe²⁺ or Fe³⁺ and H₂O₂ as media, generating hydroxyl radicals through light-assisted Fenton reactions to degrade pollutants; Heterogeneous catalytic degradation involves introducing a certain amount of photosensitive semiconductor materials, such as TiO₂, ZnO, etc., into the polluted system, combined with light radiation. Under light irradiation, the photosensitive semiconductors generate electron-hole pairs, which interact with dissolved oxygen and water molecules adsorbed on the semiconductor, producing highly reactive hydroxyl radicals (OH) with strong oxidative capabilities. TiO₂ photocatalytic oxidation technology demonstrates significant advantages in oxidizing and degrading organic pollutants in water, particularly those that are difficult to degrade.
10. Supercritical Water Oxidation (SCWO) Technology
SCWO uses supercritical water as a medium for homogeneous oxidation and decomposition of organic matter. It can decompose organic pollutants into inorganic small molecules such as CO₂ and H₂O in a short time, while sulfur, phosphorus, and nitrogen atoms are converted into sulfate, phosphate, nitrate, and nitrite ions or nitrogen gas.
SCWO has a fast reaction rate and short residence time; high oxidation efficiency, with most organic matter treatment rates exceeding 99%; simple reactor structure and compact equipment size; wide application range, suitable not only for the treatment of various toxic substances, wastewater, and waste, but also for the decomposition of organic compounds; No external heating is required, resulting in low treatment costs; it has good selectivity, as adjusting temperature and pressure can alter the physical and chemical properties of water, such as density, viscosity, and diffusion coefficient, thereby modifying its solubility of organic matter and achieving selective control over reaction products.
Supercritical oxidation technology has already been applied in industrial processes in countries such as the United States, Germany, Sweden, and Japan, but research in China has started relatively late and is still in the laboratory research stage.
